You run into the problem of differences in silver/gold/etc content, so in addition to the difference in price between gold and silver themselves, the weights and purity of coins varied. Rome was a very sophisticated place to have a monetized economy under those circumstances. A merchant had to have a keen eye and good scales. It’s should be no real surprise that the economy in the West reverted to barter eventually. Imagine having a bag of euro and dollars and kroner today without access to a computer to know their immediate value.